Various methods are used by communications systems to allow electronic devices such as computer devices to communicate and exchange signals, data and other types of messages and information. Systems such as LANs (Local Area Networks), the Internet and conventional telephone networks often link computers, phones and other devices. Another method in use today to allow mobile computers to communicate is the WLAN (Wireless Local Area Network). The I.E.E.E. or IEEE (Institute for Electrical and Electronics Engineers) 802.11 wireless networking standard, is an industry set of protocols that defines many of the standards that allow communications interoperability among the manufacturers and vendors of WLAN devices. The IEEE 802.11 standards specify several distinct OSI Physical Layer radio transmission mechanisms, such as radio frequencies, whereby signals may be transmitted over the wireless medium. A MAC (Medium Access Control) layer is also defined that organizes and controls the exchange of data packets delivering frames or messages between the communicating stations. The IEEE 802.11-19979 MAC also supports mechanisms whereby special IEEE 802.11, compliant wireless stations, called APs (Access Points) connect to a wired LAN, to bridge or connect, as a frame relay device, the wired and wireless portion of the network infrastructure. Often the wired LAN may further be connected to other networks or access the broader Internet. Mobile computers or PDAs connect to the APs wirelessly using 802.11 WLAN adapters or NICs (Network Interface Cards). These adapters may also be built directly into the devices seeking wireless connectivity.
As defined by the IEEE 802.11 standard, an AP (Access Point) is any device containing an IEEE 802.11-conformant MAC and PHY interface to the wireless medium that provides associated IEEE 802.11-compliant stations with access to the backbone infrastructure. Stated another way, the AP bridges the two network elements together to provide seamless communications from the wireless to the wired infrastructure in a bidirectional fashion.
Several IEEE 802.11 PHY (Physical layer) standards; IEEE 802.11a, IEEE 802.11b and IEEE 802.11g are showing worldwide acceptance. The 11 Mbps (MegaBit Per Second) IEEE 802.11b PHY, operating at 2.4 GHz and employing CCK (Complementary Code Keying) single carrier QPSK modulation, has been shipped since before 2000. The 54 Mbps IEEE 802.11a PHY, operating in the 5 GHz band and based on multiple carrier OFDM (Orthogonal Frequency Division Multiplex) signaling is seeing some acceptance for large company WLAN deployments. The IEEE has also standardized a combined CCK and OFDM-based extension to the 2.4 GHz 802.11b PHY called IEEE 802.11g which is gaining popularity.
With the increased popularity of mobile devices and wireless access come new applications using an increasing amount of bandwidth. In addition to an increasing number of wireless devices, many devices are becoming more mobile over time. Laptop PCs with built in wireless connectivity are replacing traditional desktop devices as the PC of choice. Users expect wireless connectivity not only throughout their enterprise and at home, but also at airports, cafes, hotels and other local “hot spots”. With the increasing number of users, newer applications hungry for higher bandwidth and additional devices and services such as voice or streaming video, it's not surprising the bandwidth needs in the wireless space are ever increasing.
One method to increase available bandwidth is to more efficiently use the bandwidth currently available. Portions of the bandwidth in IEEE 802.11 networks are allocated using CSMA/CD. Having less bandwidth allocated in this manner or decreasing the number of “collisions” which occur are methods to gain added throughput from the RF channels available. IEEE 802.11 networks also rely on scanning as a necessary mechanism to facilitate operation of the network. Using active scanning techniques, a client can search for an AP or other stations to communicate with. This process usually involves the client sending probe requests on each channel it is configured to use and waiting for responses. The client then determines which AP or station is the ideal one to communicate with. Typically most clients have single radios. Thus, when the radio is used to scan a channel not used for data transfer useful bandwidth is lost. The client station or AP can also use passive scanning. As its name implies, the station does not transmit any frames but rather listens passively for beacon frames on each available channel. The client continues to change channels at a specified interval, just as with active scanning, but the client does not send probe requests. Active scanning is the most thorough mechanism used to find APs because it actively sends out 802.11 probes across all channels to find an AP. It requires the client to dwell on a particular channel for a set length of time waiting for the probe response.
With passive scanning, the client iterates through the channels slower than active scanning because it is listening for beacons that are sent out by APs at a set rate, such as ten beacons per second. The client must dwell on each channel for a longer time duration if it is unsure of the start of the beacon interval and to make sure it receives beacons from as many APs as possible for the given channel. The client looks for different information elements such as SSID, and supported rates to aid in selecting an appropriate AP or station for communications.
There is no ideal technique for scanning. Passive scanning has the benefit of not requiring the client to transmit probe requests but runs the risk of potentially missing an AP because it might not receive a beacon during the scanning duration. Active scanning has the benefit of actively seeking out APs to associate to but requires the client to actively transmit probe requests. Depending on the implementation for the 802.11 client and the services supported one technique might be better suited than the other. For example, many embedded systems use passive scanning as the preferred method, whereas 802.11 Voice over IP (VOIP) phones and PC client cards often rely on active scanning, trading off the lost bandwidth of active probing for the shorter time interval spent in scan mode. What is clear is that all single radio systems suffer from lost useful bandwidth while implementing the scanning function. While a second radio could eliminate the lost bandwidth, the second radio would add substantial cost to the wireless system.